An optical position-measuring device of the species for generating displacement-dependent, phase-shifted scanning signals is known from the European Patent Application EP 2 765 394 A2. It includes a scanning unit having a fiber-optic scanning head, as well as a material measure that is movable relative thereto in at least one measuring direction. The fiber-optic scanning head is connected to an optical fiber, upstream of whose material measure-side end thereof, a scanning reticle is disposed. Provided at the end of the optical fiber facing away from the material measure side is a light source, as well as a detector device having a detector arrangement. The material measure is broadband-illuminated, respectively scanned by the light source, so that, for a specific wavelength, sinusoidal scanning signals result in response to a relative movement of the material measure and the scanning unit. Thus, what is commonly known as an encoding of scanning signals as a function of wavelength is performed here, i.e., the phase relations of the generated phase-shifted scanning signals are wavelength-dependent. On the detection side, the detected light must then be split into the spectral components thereof in order to generate the phase-shifted electrical scanning signals required for further processing. To this end, the detector device has splitting means for separating the sub-beams incident thereon as a function of wavelength, the splitting means including a diffraction grating. Thus, in the known device from the European Patent Application EP 2 765 394 A2, the required wavelength-dependent splitting of the detected light is carried out spectrometrically.
In this type of optical position-measuring device, the displacement of the phase relation of the scanning signals as a function of the wavelength variation constitutes an essential design parameter. The displacement is to be dimensioned to be at least large enough to make it possible to obtain three scanning signals, which are each phase-shifted by 120° (or, in some instances, four by 90°), within the available spectrum of the illuminating light. Thus, for example, if a 30 nm wide spectrum is available on the illumination side, the dispersion of the position-measuring device is then to be selected in a way that allows a wavelength variation of 10 nm to produce a phase shift of 120°. On the other hand, this means that the splitting means provided must ensure a resolution of 10 nm.
In the known device from the European Patent Application EP 2 765 394 A2, the beams, which are sent back via the optical fibers to the detector device, are first collimated by a collimator lens disposed between the optical fiber and the detector device. To ensure that sufficient optical power, for example from a LED light source, can be transmitted through an optical fiber, it is necessary that the optical fiber have a large enough numerical aperture and a large enough core diameter. Due to the finite size of the core diameter, the beam has a specific divergence following the collimation. This beam is to be subsequently split by the splitting means, which is designed as a diffraction grating, at different deflection angles into the spectral components thereof. However, different spectral components are then resolved by the diffraction grating only when the difference in the deflection angles is greater than the divergence of the beam to be deflected. Thus, a small enough divergence is needed to achieve the requisite resolving capability. To this end, the collimator lens provided between the optical fiber and the detector device must have a long enough focal length. The result is a correspondingly large diameter of the collimator lens, if no light is to be lost during the detection process. Thus, the required spectroscopic resolving capability of the detector device substantially determines the detection-side size of the position-measuring device that is provided.